When the VLA was designed, most astronomers were not overly concerned with good sensitivity to low surface brightness features, with imaging of fields of view wider than its primary beam, or with imaging at angular resolutions below that provided by the D configuration. Mosaicing had not been developed and it was believed that, in any case, such issues were better addressed by large single dish instruments. It is now recognized that compact arrays with total power capabilities fill a gap between the imaging capabilities of conventional interferometer arrays and those of large single dishes. An ultra-compact E configuration with maximum baseline lengths of a few m would provide this capability.
Given that the Arecibo telescope (for example) samples a similar aperture how, specifically, do the capabilities of an E configuration compare with those of such a large single dish? The single dish has superior point-source sensitivity--the 225-m aperture that is illuminated at Arecibo would be about 3 to 4 times more sensitive than the proposed E configuration. The basic advantage of an ultra-compact array is its imaging capability, e.g., the proposed E configuration could be roughly 10 times faster than Arecibo for survey work. Hence, the main rôle of the E configuration is to provide a fast, low-resolution wide-field imaging capability via mosaicing. The VLA also has access to 85% of the sky (unlike Arecibo, from which of the sky is visible), and will provide frequency coverage up to 50 GHz. As an interferometer, the VLA also has lower systematic errors than a large single dish; it is less susceptible to pointing errors, and ground pickup is uncorrelated between antennas, as is RFI in many cases.